In 2013, the Yougang Tang's team proposed a novel multi-drum FPSO (also can be FDPSO) made of concrete materials in the National Water Conservancy Project Simulation and Safety Key Laboratory in Tianjin University (refer to FIG. 1). Said novel multi-drum FPSO adopts the patented technology ‘Airtight Air Pressure Communicating Type Ballast Seawater and Crude Oil Constant-Specific Mass Flow Replacement Process’ proposed by the specialist Zhi-Rong Wu in China National Offshore Oil Corporation as a design basis and realizes the optimization on the hydrodynamic performance of the novel floating body [1].
This technology mainly lies in that an oil storage compartment intercommunicates with the top of a ballast seawater compartment through a pipeline, the compartment body is airtight, but prefilled with nitrogen having a certain pressure, and a crude oil inlet pump and a seawater unloading pump as a group are in linkage with a crude efflux pump and a seawater ballast pump as another group, respectively, so as to realize constant-specific mass flow replacement. This technology aims at ensuring unchanged weight of the floating body, unchanged scantling draft and small amplitude of vertical variation of center of gravity in the oil storage and efflux process.
This oil storage technology has the outstanding advantages that the floating body is always kept at the same stable state, and the optimization of the hydrodynamic performance is realized; secondly, due to the separate storage of ballast water and crude oil, the pollution to the marine environment is reduced to a great extent; but simultaneously has the following unignorable defects.
Firstly, the total mass tremendously changes and the hydrodynamic performance has huge difference under full-load and unloaded working conditions, both of which are hard to reconcile in the design process. In today's increasingly intensified and multi-functionalized maritime work development, the importance of effective loading capacity is more and more prominent to become a very important performance and economic indicator for a platform. According to this technology, the platform is required to have to reserve enough fixed ballast in order to make the constant draft reservation space ratio approach to 4:5, which itself is extremely wasteful to the loading capacity of the platform, and thus greatly reduces the economy of the platform.
Secondly, the cost input of equipment, such as a nitrogen generating device, a pressure control device and a storage tank with an inner layer and an outer layer, of this patient is unignorablely high due to the complex process; and further, the use reliability of the equipment is reduced and potential risk of prolonging the downtime exists. The schematic diagram of devices is as shown in FIG. 2.
In 2006, in new concept FPSO/FDPSO cooperatively studied by France DORIS Corporation and Technical University of Lisbon, an innovative water-oil mixing storage technology [2,3] is adopted in order to keep the platform at a constant floating state and control the waterplane area (referring to FIG. 3).
According to the principle that densities of crude oil and water are different, crude oil is located at the upper part of the oil storage compartment, and water is located at the lower part of the oil storage compartment. In the production process, oil entering the oil compartment expels water with the same volume to sea through a water treatment system. The water treatment system ensures the cleanliness of water drained. On the other hand, in the output process of oil, the volume of output oil is filled up by seawater in the sea entering the oil compartment. In order to keep draft constant, ballast water with the same weight must enter or must be removed to compensate the volume change of oil and water, which can be achieved by a ballast water compartment supplied by each column. In the loading and unloading process, any sudden change of the oil-water compartment can be restricted by an atmosphere regulating compartment, and the interface between oil and water can freely fluctuate in the atmosphere regulating compartment. In the loading process, oil is filled into an oil buffer compartment which is connected with four oil storage compartments in a caisson. When the oil level in the buffer compartment rises, water at the lower part of the oil storage compartments can be drained to a water buffer compartment by virtue of increased height of an oil column. Water in the water buffer compartment is pumped into the water treatment device through a submerged pump and is then drained to the sea. The design concept map of the oil storage compartments is as shown in FIG. 4.
The oil storage compartment has the advantages that the total mass of the platform can be effectively controlled not to change to further keep the floating state unchanged, the loading capacity of the platform is sufficiently utilized and favorable economy is achieved, but has serious defects as well.
Firstly, at the stage of coexistence of water and oil, the interface between water and oil can continuously change along the movement of the floating body, it is inevitable to cause the emulsification problem on the inference in the sway process since oceanic conditions change at every moment, and therefore, the input/output speed must be strictly controlled to prevent water and oil from mixing. Since the emulsification phenomenon is serious resulting from complex oceanic conditions, the crude oil input/output work cannot be continued until the interface between oil and water is clear after standing a period of time, which seriously restricts the sufficient exertion of the processing capacity of the platform and reduces the operation efficiency, and even so, the emulsification problem cannot be completely eradicated yet.
Secondly, since water and oil are stored in a mixed manner, ballast water of certain height will be reserved in the compartments for preventing crude oil from entering the water buffer compartment according to the design requirements even under the condition of full load of crude oil. Water and oil are inevitably fused to each other in this longer period from oil production to crude oil unloading, and there will be a large number of salts in seawater to enter crude oil, resulting in increase of salt content of crude oil and reduction of crude oil quality.
Then, the crude oil needs to be stored by heating under normal conditions due to high solidifying point, however, in view of coexistence of water and oil, heat exchange inevitably exists to cause loss of a lot of heat along with drainage of ballast water, which will further cause waste of a lot of heat energy and increase of the operation cost of the platform and simultaneously bring unnecessary troubles for subsequent transfer of crude oil.
Finally, in order to protect the marine environment, the water and oil mixing storage technology brings great pressure to subsequent ballast water treatment. Differing from a washing process flow, the oil content of the ballast water will be greatly increased according to this technology, and it means that treatment equipment with higher capacity is needed to meet the technological flow requirement, so just one factor of restricting the operation efficiency is added.